1. Field of the Invention
The present invention relates to a device for monitoring multiplexing of CPS (Common Part Sublayer) packets and a method of controlling cell assembly. More particularly, the present invention relates to a method and an apparatus for controlling cell assembly times set connection by connection as in newly proposed AAL2 (ATM (Asynchronous Transfer Mode) Adaptation Layer 2).
2. Description of the Background Art
It is a common practice with an intra-corporation or similar private network to compress speech signals to be sent. ATM technologies have customarily been applied to speech communication for setting up standards capable of meeting the ever-increasing demand for efficient communication and the integration of speech and data networks.
ATM allows information to be sent at a rate as high as megabits per second. With ATM, it is possible to divide, e.g., compressed information before assembling a cell with AAL or to restore the original format of data constituting a cell with AAL. Today, four different types of AAL are available, i.e., AAL1, AAL2, AAL3/4 and AAL5 assigned to a particular kind of transmission service each. On the other hand, speech communication is effected at a rate as low as several kilobits per second and variable by speechless compression. When speeches are constructed into an ATM cell by AAL1 having a standardized protocol with a fixed transmission rate, a delay ascribable to cell assembly increases with an increase in compression rate. While some different schemes have heretofore been proposed to reduce the delay, they have some problems including a decrease in efficiency and inconstant data size left unsolved.
To assemble a cell with high efficiency, it is necessary to construct packets by adding headers of speech data and then multiplexing such packets of a plurality of channels to assemble a single composite cell (so-called mapping). AAL3/4 lacks the function of multiplexing a plurality of channels while identifying the individual connection, while AAL5 undesirably increases the overhead of a protocol and complicates processing. This is why AAL2 has been newly standardized.
In accordance with AAL2, a format is made up of a CPS-PDU (Common Part Sublayer-Protocol Data Unit) header or STF (Start Field) following an ATM header, and a CPS-PDU. A CPS packet in the CPS-PDU is made up of a three-octet CPS packet header and a variable-length payload. The variable-length payload, like an ATM cell payload, is a data unit having forty-eight octets. A plurality of CPS packets are multiplexed and mapped in the variable-length payload.
Trends and studies relating to standardization taking account of multimedia environments are discussed in Takenori Okutani xe2x80x9cStandardization trend and technical issue of VTOAxe2x80x9d, TECHNICAL REPORT OF IEICE, IN96-36 (1996-07), pp. 35-40, Hisakazu Tsuboya xe2x80x9cApplication of ATM to Mobile Communications Network-Technical Subjects of ATM for Low Bit Rate Voice-xe2x80x9d, TECHNICAL REPORT OF IEICE, IN96-37 (1996-07), pp. 41-46, and Hisahi Usijima et al. xe2x80x9cA study on voice-band signal processing in private ATM networkxe2x80x9d, TECHNICAL REPORT OF IEICE, IN96-38 (1996-07), pp. 47-54.
While the above multiplexing and mapping is, in principle, completed within a preselected cell assembly time in order to reduce the delay, the delay sometimes increases in practice. In light of this, a cell transmitting station counts a cell delay with, e.g., a timer. When the delay exceeds a preselected value before the completion of a cell, the transmitting station fills the incomplete portion of the cell with padding and then sends the resulting partially filled cell.
In a system of the type sending an ATM cell on the elapse of a cell assembly time and the completion of multiplexing, a particular cell assembly time should preferably be assigned to each connection from the efficient CPS packet transfer and limited allowable delay standpoint. However, when the cell assembly time differs from one connection to another, it is necessary to control CPS packets by monitoring the cell assembly time connection by connection. Moreover, it is necessary to determine whether an input CPS packet is the first CPS packet or whether it is a CPS packet being multiplexed.
It is therefore an object of the present invention to provide a monitoring device and a cell assembly control method capable of controlling a cell assembly time while monitoring the multiplexing of the individual CPS packet.
In accordance with the present invention, a device for monitoring a cell assembly time of a packet multiplexing circuit which multiplexes and maps a plurality of packets received via a packet receiving circuit and each consisting of a header and user information to thereby assemble a cell having a fixed length for providing a variable-speed service, the cell being sent via a packet transmitting circuit, includes a timepiece for feeding a cell assembly start time based on a value occurring when a packet is input to the packet multiplexing circuit for the first time and a reference time of the device. An information storage stores, connection by connection, connection control information defined on a protocol layer basis and set between the same layers for indicating a relation between terminals to communicate with each other. A monitoring circuit monitors, based on the connection control information, the cell assembly start time and the reference time, a time for ending packet multiplexing connection by connection, and causes the packet multiplexing circuit to execute processing in accordance with the result of monitoring.
Also, in accordance with the present invention, a cell assembly control method for controlling a cell assembly time necessary for multiplexing and mapping a plurality of received packets each consisting of a header and user information to thereby assemble a cell having a fixed length to be sent for providing a variable-speed service begins with a step of feeding a cell assembly start time based on a value occurred when a packet is input for the first time and a reference time for cell assembly. Connection-by-connection control information defined on a protocol layer basis and set between the same layers for indicating a relation between terminals to communicate with each other are read out of an information storage. Whether or not an input packet is input for the first time is determined on the basis of activity information included in the connection control information. If the input packet is input for the first time, the activity information is updated by a value indicative of a multiplexing and mapping operation. At the same time, a time for ending multiplexing is calculated for a connection to execute multiplexing and stored in the information storage. The multiplexing and mapping operation is continued if the above decision on the input packet shows that multiplexing is under way. The time for ending multiplexing assigned to the connection executing multiplexing and the times fed at the first step are compared. If the time for ending multiplexing is reached, the activity information is updated with a value indicative of the end of the multiplexing and mapping operation and the stored in the information storage. The transmission of a cell then controlled.